A multilayer film which is formed by laminating TiO.sub.2, SiO.sub.2 etc., is used e.g. for a color separation prism in video cameras. This film has the function of separating light coming from an object into light with specific wavelengths, such as red, blue, and green.
Recently, optical devices are required to show high performance so that they can keep pace with advanced electronics. This is especially true in terms of multilayer films used e.g. for a color separation prism which requires uniform thickness and quality.
There are two conventional methods used to control optical thickness when this kind of multilayer film is formed: a monochromatic photometric method and a dichroic photometric method. In the following, an example of the monochromatic photometric method is explained with reference to FIG. 22.
Film forming chamber 61 is kept in high vacuum condition through exhaust pump 60. EB (electron beam) gun 62 disposed in this chamber melts evaporation material 63 by heat. Evaporated particles 64 reach substrate 66 which is disposed on substrate holder 65 and form a film. At the same time, after passing through hole 67 of substrate holder 65, evaporated particles 64 reach monitoring substrate 68 and form a film. Light radiated with a specific wavelength from light source 69 reaches the film formed on monitoring substrate 68, and reflected light quantity is detected at detector 70. Since the reflected light quantity changes according to the refraction index or thickness of the film formed on monitoring substrate 68, computer 71 reads the reflected light quantity while the film is being formed. As soon as the reflected light quantity reaches a predetermined value, shutter 72 closes and ends the evaporation process. In this way, the first layer is completed.
In the next phase, evaporation material 63 is replaced with another material, and monitoring substrate 68 is also replaced with a new substrate. The same process which took place in forming the first layer follows, i.e. EB gun 62 melts evaporation material 63 by heat, and the second layer is formed on substrate 66. By repeating this operation many times, a multilayer film having relatively uniform thicknesses is formed.
However, the above-mentioned conventional film forming method has the following problems.
Since light quantities reflected and transmitted from the film are used for this operation, it is extremely difficult to control with high accuracy of less than 0.5% due to light noise etc. from the light source and evaporation source. The quantities of reflected and transmitted light changed periodically according to optical thickness which was calculated from multiplying the refraction index by the thickness of film. Accordingly, it was relatively easy to control the optical thickness by means of this periodicity. However, it is difficult to control the thickness with high accuracy since the thickness changes even if the optical thickness is constant. This happens when conditions, such as degree of vacuum or temperature, cause subtle changes, and the refraction index of the film changes accordingly.
In the past few years, the requirements for optical devices have become extremely rigid. Therefore, the optical thickness of the prior art material is not accurate enough, and a more accurate controlling method is desired. Furthermore, there was a major problem in quantity production. When a multilayer film consisting of tens of layers is formed, the entire products in the batch are no good if the thickness of one layer fails to be controlled for some reason.